1. Field of the Invention
The present invention relates to a lithographic apparatus, an illumination system, a controller and method to control an output of a pulsed source of radiation.
2. Description of the Related Art
A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g., including part of, one, or several dies) on a substrate (e.g., a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Conventional lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at once, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
In a lithographic apparatus, as well as in many other applications, a surface such as a substrate surface or a wafer surface is to be illuminated by a pulsed source of radiation. The pulsed source of radiation, such as a pulsed laser, provides for a series of pulses, e.g., at a certain pulse repetition rate. During an illumination, the substrate or other object, which is to be illuminated, is moved such that with each pulse a different part of the surface of the substrate or other object is illuminated. Commonly, surfaces which are illuminated by subsequent pulses will show a certain amount of overlap. Thus, each location of the surface of the substrate which is to be illuminated is generally provided with optical radiation from at least two pulses. Commonly, a relation between the pulse repetition frequency, a size of a window of the substrate or other surface which is to be illuminated, and a scanning speed of the substrate or other to be illuminated object, is chosen such that each point of the surface of the substrate or other to be illuminated surface is illuminated by a plurality of pulses. Due to physical constraints, pulse energy of the pulsed source of radiation may show a certain amount of deviation. In other words, an energy provided by subsequent pulses may differ to a certain extent. Commonly, however, it is desirable to provide a homogeneous illumination, i.e., to arrange that each point on the surface of the substrate or other to be illuminated object, is provided with a substantially same dose of radiation. For this reason, a controller may be provided which drives the pulsed source of radiation thereby making use of a pulse energy of the source of radiation at previous pulses. To accomplish this, the controller may comprise a feed back loop incorporating an integrator. By the integrator, a total dose of pulses of radiation at a certain location may be stabilized to a certain extent by the integrative action of the controller, which may easily be understood as subsequent pulses illuminating a certain point at the surface of the substrate or other object will add to form a total dose at that point.
In more detail, a standard deviation SD of the integrated dose at any point of the wafer when the laser is controlled by the above described controller may be expressed as:
      SD    ⁢                  ⁢          (      output      )        =            SD      ⁢                          ⁢              (        laser        )                    N      ⁢              √                                  ⁢                  (                      n            /            2                    )                    
wherein SD (output) represents a standard deviation of the output of the laser when in the control loop, SD (laser) represents a standard deviation of the pulsed source of radiation as is, thus without the controller, and N represents a number of laser pulses in a slit which is used to create a window of illumination onto the substrate or other surface, and n represents a number of pulses in a slope of the slit profile. Thus, it can be easily seen that a standard deviation of the output may be reduced by reducing the standard deviation of the laser as well as by increasing the number of pulses in a slit, e.g., by increasing a pulse repetition frequency or by decreasing a scanning speed with which the substrate or other object is scanned.
The principle of controlling as described above has been used for a long time in many applications. In fact, it is believed by the person skilled in the art that an improvement to the controller as described above may be difficult.